Generally, integrated circuits (IC) are fabricated on a relatively hard semiconductor substrate, such as silicon (Si), gallium arsenide (GaAs), and others. These relatively hard semiconductor substrates, when properly prepared, can be made to be extremely smooth. The smoothness of the semiconductor substrates (or simply substrate) is one of the primary factors in determining the manufacturing process that can be used on the substrate, i.e., the density or feature size of the manufacturing process. The smoothness (or roughness) of a substrate can be specified numerically, usually a number indicating an average dimension of various pores, valleys, pits, cracks, and other imperfections on the substrate. If a substrate is relatively rough, then fabricating ICs with feature sizes that are smaller than the average dimensions of the imperfections on the substrate will likely yield a large number of defects in the ICs.
The defects in the IC will likely be due to the imperfections in the substrate not permitting the proper formation of the structures of the IC. For example, during the formation of conductive lines (using conductors such as aluminum or copper along with other possibly conductive materials) in a valley or crack in the surface of the semiconductor may result in the conductive material flowing into unintended areas. The conductor in the valley (or crack) can then cause an electrical short circuit with other portions of the IC. The short circuit could then prevent the IC from operating as intended. In addition to short circuits, imperfections can cause breaks in a conductive line, resulting in an electrical open circuit. Imperfections in the substrate can also result in certain desired features being malformed and hence not likely to operate as intended.
A move is being made in the semiconductor industry towards substrate materials with low dielectric (low k) coefficients. These low k substrates, such as substrates made from a combination of an organic substance (or substances) with silicon, tend to be soft and/or fragile. Additionally, they can be more sensitive to heat and pressure than silicon (and other hard semiconductor substrates). Furthermore, the k-value of these low k substrates can be affected by the size and number of pores in the substrate. Therefore, should the size and number of pores in a low k substrate be modified in some fashion, the k-value of the substrate may be changed. This can have a detrimental effect on the desired properties of the substrate.
A common step in the fabrication of an IC on a semiconductor substrate involves the cleaning of the substrate at various points in the fabrication process. The cleaning step can be used to eliminate residue from the previous (or several previous) fabrication steps from the substrate. For example, the cleaning step can remove post-etched and/or post-ashed residue from the substrate. If these residues are left on the substrate, the k-value of the substrate may be altered and/or subsequent fabrication steps may not be able to be performed in an optimal manner due to contamination, perhaps resulting in defective ICs.
One way to clean a substrate is to use a supercritical fluid at high pressure. A supercritical fluid is any substance that is at a temperature and pressure that is above its critical temperature and critical pressure. Examples of supercritical fluids that can be used to clean substrates include but are not limited to: nitrogen, argon, xenon, carbon dioxide, propane, ammonia, methanol, water, and others. The supercritical fluid, injected onto the substrate at high pressure, can remove the unwanted material. Then, when the substrate is brought back down to normal pressures and temperatures, the supercritical fluid will typically leave very little (or no) residue on the substrate and the fabrication process may continue.
A disadvantage of the prior art is that the supercritical fluid is applied at high pressure, wherein the pressure must exceed the critical temperature and pressure of the fluid. The high pressure can create cracks, holes, and other imperfections in the substrate, especially if the substrate is made using a low k material that tends to be softer and more fragile than a typical semiconductor substrate. Additionally, the high pressure can enlarge and intensify existing imperfections in the substrate. Such damage to the semiconductor substrate may result in defective ICs.